I’ve wanted a smart thermostat for a while but live in an apartment with a fixed (no remote control), mains powered thermostat. Unfortunately all of the smart thermostats are designed for American HVAC systems which mostly use a 24vac system. I ended up buying an ecobee3 lite and a few things to make an adapter.
3x 24vac relays to switch the mains (240v) control signals.
1x 24vac power adapter to power the ecobee and the relays.
1x fuse and fuse holder for safety.
Basically one relay was wired up to switch each of the heating, cooling and low fan speed control signals. The 24vac side was attached to the ecobee to the relevant terminals (cooling - y, heating - w, fan - g). The 24vac relay was hooked up to the ecobee “R” and “C” terminals.
After it was hooked up and power was turned back on the ecobee booted up. It asked me if it detected the correct wiring (yes). Then I connected it to the wifi and it spent a few minutes calibrating it’s sensors and then it sprung into action cooling our apartment.
Version 2 will have a small pcb with everything mounted on It for better safety and reliability. I’ll also include a small 24vac transformer so no external power adapter will be needed (that’s how it’s currently connected).
Here’s the old thermostat:
Thermostat installed (with adapter behind, ie inside wall):